This invention relates to an objective lens drive device in an optical disk device etc.
When data is written to and read from a disk in an optical disk device, it is necessary that the light which is used for writing and reading is focused on to the disk, and that the spot produced by this light follows a track. Normally, for the purposes of this focus control and tracking control, an objective lens that directs and focuses this light on to the disk is arranged to be moved in the focusing direction, i.e. perpendicular to the disk, and also in the tracking direction, i.e. perpendicular to the track.
Various methods for tracking control have been proposed. In an optical video disk device, for example, "three-beam" system is adopted. In this system, apart from a single main beam for data reading, two auxiliary beams are used for tracking. If this system is applied to a device for writing data on disks, there is a risk that data may be destroyed by the auxiliary beams. Accordingly, in a device that performs not just reading but also writing, a "far field" system ("push-pull" system) is often adopted, wherein the single beam for data writing and reading is also used for tracking. However, this system has a drawback that a component corresponding to the variation in the light amount that is produced by the movement of the beam is superimposed on the tracking error signal.
A method for solving this problem has been disclosed in Japanese Patent Application Laid-Open No. 198436/1986. In this method, the light that is output from a light source is directed on to the disk through an objective lens. The reflected light is received by a first photodetector that is divided into two photodetection elements in the direction parallel to the track. The difference in outputs of this pair of photodetection elements is used to generate a far field tracking error signal T.sub.1. Additionally, the light that is output from the light source is divided, and part of it is input to a slit that is moved in the tracking direction in association with the movement of the objective lens. The light which is output from the slit is received by a second photodetector fixed at a prescribed position. The second photodetector is also divided into two photodetection elements. From the difference in outputs of this pair of photodetection elements, a signal T.sub.2 is generated, corresponding to the fluctuation in position of the objective lens (light spot on the disk) in the tracking direction. In the output signal T.sub.1 of the first photodetector, the true tracking error signal T.sub.3 and the objective lens position fluctuation signal T.sub.2 are superimposed. Consequently, the true tracking error signal T.sub.3 is derived from the difference between the outputs of the first photodetector and the second photodetector.
If this system is applied to an objective lens drive device as disclosed in for example Japanese Patent Application Laid-Open No. 18631/1987, the construction shown in FIGS. 4, 5 and 6 would be obtained.
In these figures, 1 is a main frame of an objective lens drive device, 2 is a support shaft which is fixed on to main frame 1, 3 is an objective lens that focuses and directs a light on to a disk (not shown), forming a light spot, and 4 is an objective lens holding member that supports objective lens 3 in a manner such that its optic axis is parallel with support shaft 2, being freely slidable along and rotatable about support shaft 2. 5 is a coil for control in the focusing direction, being wound on coil bobbin 6 which is formed at the periphery of holding member 4. 7 is a coil for control in the tracking direction, being fixed on the coil 5. 8 is an outside yoke facing the outside of coil 5 and being fixed to main frame 1. 9 is an inside yoke that is arranged opposite outside yoke 8. 10 is a permanent magnet for control in the focusing direction, forming a magnetic circuit with yokes 8 and 9. 11 is a yoke fixed to main frame 1. 12 is a permanent magnet for control in the tracking direction, being fixed to yoke 11 and facing the outside of coil 7. Thus coil 7 is arranged in a magnetic circuit for tracking provided with yoke 11 and permanent magnet 12. Main frame 1 has a hole 13 to allow passage of light for writing and reading, and a hole 14 to allow passage of light for position detection, which is divided from the light for writing and reading. Holding member 4 is provided with a slit 15 on a straight line (on a radius) in the diametric direction passing through the center of rotation of support shaft 2. A photodetector 16, which is divided into two photodetection elements, is fixed on the underside of a cover 17, in such a way that the line of its division is contained in a plane including the center line of slit 15 when holding member 4 is in a reference position. Photodetector 16 constituting the aforementioned second photodetector (for position detection of the objective lens) is divided into two photodetection elements 16a and 16b by the dividing line. Cover 17 protects the above components, and is provided with a hole 18 through which the light that is output from objective lens 3 passes.
The operation will now be described. When misalignment of the focal point of light incident on to the disk through the objective lens is detected by means, not shown, a current corresponding to the amount of this misalignment (focus error) flows in coil 5, and acts, with the magnetic field produced by permanent magnet 10, to drive holding member 4 (objective lens 3) in the direction of the axis of support shaft 2. An elastic member (not shown) is provided between holding member 4 and yoke 11 or another fixed part on main frame 1, and focusing control is performed about the position of this elastic member as center.
A signal accurately corresponding to the misalignment, with respect to the track on the disk, of the spot of light projected from objective lens 3 is generated utilizing the outputs of aforementioned first (tracking error detecting) photodetector, not shown, and photodetector 16, which is the second photodetector. A current corresponding to this amount of misalignment (tracking error) flows in coil 7, and acts with the magnetic field of magnet 12, to produce rotation of holding member 4 (objective lens 3) about the axis of support shaft 2. Tracking control is performed about the position of an elatic member as center, in the same way as in the case of focusing control.
The light for position detection is parallel light and is guided into hole 14 of main frame 1 and output from slit 15 provided on a straight line passing through the center of rotation of holding member 4 that moves in unison with objective lens 3. Photodetection elements 16a and 16b are disposed in a manner such that the dividing line of photodetector 16 passes through the center of the output light of slit 15 when objective lens 3 is in the reference position. That is, if we represent the output signal of photodetection element 16a by A, and the output signal of photodetection element 16b by B, objective lens is adjusted in the reference position in which A=B. If objective lens 3 moves in the tracking direction (direction of rotation), A or B becomes greater than B or A, so the amount of misalignment from the reference position of the objective lens can be detected.
The objective lens drive device described above has to be adjusted such that the beam that passes through the slit is symmetric with respect to the dividing line of the photodetector when the objective lens is in the reference position for position detection in the tracking direction. This results in the problem that the cover must be adjusted relative to the main frame after mounting the photodetector on the cover, which is troublesome and time-consuming.